ARSENIC IN WESTERN OREGON: SUMMARY OF STUDIES BY PORTLAND STATE UNIVERSITY STUDENTS

This summarizes research by Portland State University students examining the natural background concentrations of arsenic in the soils of western Oregon. Original soil samples, collected by Ashbaugh (1995), Khandoker (1997) and Douglas (1999) were retested (2010) at a detection limit of 0.20 ppm by ICP-MS, as were samples from new sites by Ricker (2013) and Hurtado (2016). All new results were used in analysis of arsenic in northwest Oregon by Ricker (2013), and southwest Oregon by Hurtado (2016) in order to better ascertain the natural levels of arsenic in undisturbed soils.

Highest arsenic values in the A and B horizons, respectively, are 13.9 ppm (site C4) and 20.4 ppm (site P4) in the Ricker study, and 63.2 ppm and 85.4 ppm (site HH14) from the Hurtado study. Mean and standard deviation for northwest Oregon are 6.09 ± 2.65 and 10.26 ± 4.65 for the Coast Range, 3.59 ± 3.72 and 3.25 ± 2.23 for Willamette Valley and 2.45 ± 4.29 and 3.12 ± 4.47 for the Cascade Range (Ricker, 2013). In southwest Oregon, highest and average concentrations with standard deviation for each province, respectively, are: South Willamette Valley, 85.4 and 21.99 ± 10.5 ppm; Klamath Mountains, 45.4 and 5.42 ± 3.0 ppm; Cascade Range, 11.9 and 2.76 ± 1.2 ppm; Coast Range, 10.6 and 5.15 ± 3.0 ppm; Basin and Range, 2.32 and 1.29 ± 0.5 ppm; High Lava Plains, 1.5 and 1.20 ± 0.2 ppm (Hurtado, 2016).

To test distribution and variance of arsenic in the A and B soil horizons, five samples were taken from the A and from the B horizons at a randomly selected site (Ricker, site TR06; Hurtado, site HH11). Mean and standard deviation for site TR06 was 4.82 ± 1.41 for the A horizon and 10.96 ± 0.47 for the B horizon, and 3.74 ± 0.44 for the A horizon and 4.53 ± 0.39 for the B horizon for site HH11.

Lastly, Ricker (2013) and Hurtado (2016) ran statistical tests to look at potential relationships between arsenic in soils and other environmental factors. Analysis of Variance (ANOVA) tests by Ricker (2013) indicate that the arsenic content of the Marine Sediments and Sedimentary Rocks groups are distinguishable from other tested groups. Regression forests by Hurtado (2016) showed greater arsenic concentrations within Alfisol and Ultisol/Alfisol and Vertisol soil orders, at lower elevations below 1,207 meters, and within soils from sedimentary, mixed volcanic/sedimentary and unconsolidated rock types.